Add toString() method

@Override
public String toString()
Returns a String representing this Tree. The returned String
is in the same format as the parse method expects as input.
For example, for the Tree:
                            one
                            / \
                           /   \
                         two  three
                         		 /\
                         		/  \
                            four five
                             /\
                            /  \
                          six seven
toString method will return the below String,
"one(two three(four(six seven) five))"